Printing presses have been developed capable of producing quality reproductions on paper webs moving at very high speeds through the press. Speeds of the order of one thousand feet per minute are frequently employed.
When a web printing material is moving through a press at such high speed, care must be exercised that the various press components are properly aligned and that the web's integrity is maintained. If a fault occurs such as a break or a reduction in web tension below an acceptable level, the rapidly moving web is likely to jam the printing press, thus causing damage and undesirable lengthy down-time.
Automatically actuated controls to detect such faults and provide immediate protective action such as stoppage of the machine and severing of the web at the proper place have been proposed and are well known.
For example, in the U.S. Pat. No. 3,928,844 to Meihofer, a control is described as capable of automatically detecting a fault and responding with immediate corrective action. The Meihofer patent describes an electronic control for sensing the disappearance of web from a station where it should be in the processing machine, and responding by assuring an immediate machine stoppage and appropriate web severing action.
In a practical press operation, however, it is frequently necessary to change the set-up such as threading of the web past different rollers and thus different stations and accommodating for different web widths. When a set-up change is made by the operator, certain web sensing stations may have to be activated and properly positioned in operative engagement with the web while others are locked out. After a set-up change, the operator may test the operation of the press at low speeds and then resume high-speed printing.
In a large printing press many stations may be employed with a large number of web sensors and web severers. A web sensing head might be defective or an operator error can be made by threading the web past a web sensor which the operator has actually locked out. In such case a prior art press control such as Meihofer would still permit the press to run. The resulting damage to the press and downtime can be very costly.